1. Field of the Invention
The present invention relates to a surface acoustic wave filter and a method of producing the same.
2. Description of the Related Art
As with conventionally used large scale integrations (LSIs), various packages having external dimensions that are approximately the same as those of chip size packages (CSPs) have been used for surface acoustic wave filters, that is, device chips have been provided.
For example, a surface acoustic wave filter 1 shown in cross-section in FIG. 1A includes a device chip 10x mounted to a mount board 2 having an external terminal (not shown) by a flip chip pound mounting method. The device chip 10x is sealed with a resin film 6 such that a space is provided at a vibratory portion (surface acoustic wave propagation portion) of the device chip 10x. The device chip 10x includes a wiring pattern provided on one surface 11a of a piezoelectric substrate 11x which includes, for example, pads 16 and comb electrodes (IDTs) 14. Lands 3, which are metallic portions electrically connected to the external terminal of the mount board 2, and the pads 16 of the device chip 10x are electrically connected through bumps 4, such as metallic bumps including Au bumps or solder bumps.
The surface acoustic wave filter sealed by the resin film 6 in this manner is typically fabricated by a heat press method as shown in FIG. 2. More specifically, after mounting a plurality of device chips 10x to one board aggregate 2x by the flip chip pound mounting method, the device chips 10x are covered with the resin film 6. While heating the resin film 6, the resin film 6 is pressed in the direction of arrows 9 with a hot press 8 or by roll lamination. The resin film 6 softened by heat is pressed in contact with the mount board 2x at spaces between the device chips 10x that are adjacent to each other, and the device chips 10x are embedded in the resin film 6. By simultaneously dicing portions of the aggregate board 2x and portions of the resin film 6 between the adjacent device chips 10x, packages are severed into individual packages (see, for example, Japanese Unexamined Patent Application Publication No. 2003-17979 (Patent Document 1) and Japanese Unexamined Patent Application Publication No. 2003-32061 (Patent Document 2).
However, when the resin film 6 is pressed with the hot press 8 or by roll lamination, air enters between the resin film 6 and peripheral surfaces 12x extending between a pair of principal surfaces of each piezoelectric substrate 11x. As a result, large voids 6x are formed near portions at which the resin film 6 is in close contact with the board aggregate 2x, that is, the mount board 2.
More specifically, since the peripheral surfaces 12x of each piezoelectric substrate 11x are perpendicular to the pair of principal surfaces, at the initial stage of heat pressing the resin film 6, the resin film 6 is separated from the peripheral surfaces 12x of each piezoelectric substrate 11x. As a result, as shown in FIG. 1B, the voids 6x are formed due to air entering the corners or longer sides of each rectangular piezoelectric substrate 11x. 
When such large voids 6x are formed, the sealing width of the surface acoustic wave filter 1 (that is, the size of a portion at which the film 6 and the mount board 2 are in close contact between the outside and the internal space of the surface acoustic wave filter 1) is reduced. Therefore, the surface acoustic wave filter 1 is less reliable, particularly in terms of moisture resistance.